Although cerebral ischemia and stroke commonly occur in patients with atherosclerosis, the specific effects of atherosclerotic vascular abnormalities are not reflected in most animal models of stroke. This proposal addresses the effects of atherosclerosis and endothelial dysfunction on cerebrovascular function, reactivity, and response to ischemia, using Western diet-fed apoE knockout mice as an animal model. We have found that apoE knockout mice show decreased resting cerebral blood flow, impaired response to whisker stimulation, and impaired autoregulation, while the response to hypercapnia is relatively preserved. In addition, apoE knockout mice have substantially larger infarcts after middle cerebral artery occlusion than do wild-type mice. We propose three specific aims: Aim 1 tests the hypothesis that hypercholesterolemia and endothelial dysfunction alter cerebrovascular anatomy, physiology (hypercapnia, whisker stimulation, and autoregulation), and pharmacology (pial arteriolar response in cranial window studies) of apoE knockout mice. Aim 2 examines the effects of atherosclerosis and endothelial dysfunction on the response to focal ischemia. We will use high-resolution laser speckle imaging to generate real-time maps of temporal and spatial pattern of cerebral blood flow, and quantitate effects on leukocyte-endothelial cell interactions. Aim 3 tests whether abnormalities in NO generation underlie cerebrovascular abnormalities in apoE knockout mice. We will determine whether genetic or pharmacologic interventions directed at augmenting endothelial NO production will restore cerebrovascular reactivity, and ameliorate the response to ischemia. These studies will establish the link between abnormalites in NO generation, the abnormal responses seen in Aim 1, and the response to ischemia studied in Aim 2. Together, these experiments outline how atherosclerosis and endothelial dysfunction affect the cerebrovasculature, and whether pathophysiologic mechanisms operative in the peripheral circulation also apply to the cerebrovasculature. They also provide proof of principle for approaches that directly target cerebral vascular dysfunction for the treatment and prevention of stroke.